This invention relates to making a container. More particularly, it relates to monitoring sheet being fed to a multi-die cupper press or can end press for defects and interrupting the operation of a die set in the presses when a portion of the sheet having a defect is within the die set and thereby prevent a cup from being drawn or an end from being formed with the defective sheet.
A can body used in a two-piece can is either drawn or drawn and ironed. In either case, the first step in the manufacturing process is to feed sheet stock from a coil to a draw press where the sheet is blanked and drawn to finished size or drawn to form a cup for a later redraw and ironing. One very large market for drawn and ironed can bodies is for carbonated beverage containers. The market for such cans has steadily grown, and accompanying that growth has been an emphasis on reducing the thickness of metal in each container and increasing the production rate. In a modern can plant, the press to make the cup, called a cupping press or cupper, may typically have 12 identical die sets to make 12 cups with each stroke of the press and operate at speeds of 120-130 strokes/minute. It is of primary concern to the can manufacturer to make defect-free cups while running the cupper without interruption.
As a separate operation, sheet stock is fed from a coil to a stamping or drawing press where the stock is blanked, and the blank is then formed into a can end which is attached to the can end after it has been filled. As is the case with a cupper, the end shell press typically has multiple die sets, such as 22, for example, and may operate at speeds of 125-150 strokes/minute.
Heretofore, a press, whether a cupper or end shell press, has been shut down whenever more than anomalous or sporadic defective cups or ends are being produced in order to use the press efficiently and identify the reason for the defect. One source of defective cups or end shells is a defect in the sheet which manifests itself as an unacceptable flow or defect in the can body or end shell. The sheet may be out of tolerance in thickness or off-temper, for example. The defect may be only cosmetic in nature, such as streaks, for example. Other defects, however, such as slivers, rolled-over scratches, gouges, pinholes, rolled-in metals or inclusions, for example, may result in producing cups and end shells having flaws which affect their performance. A defect may extend across the entire width, but it is more likely that it appears in line with only one or two die sets. When a defect is encountered, it is often not possible to determine the extent of the defect through the coil, but rather than hazard the risk of producing a substantial number of defective cups, the usual practice is to shut the line down, remove the coil having the defect and replace it with a new coil. The removed coil would then be set aside and held for inspection by the coil manufacturer. Depending upon the nature of the defect and its location with respect to the beginning or end of the coil will determine whether additional material would be run off of the coil in the can plant in the hope that the defect extended only a short distance into the coil or whether it would be returned to the manufacturer. In any event, shutting down a can line because of defective sheet is disadvantageous and expensive for both the can maker and sheet manufacturer.
The impact of producing defective parts, however, may be substantially greater than shutting down a line and the unnecessary scrapping of good material. This is particularly true with respect to making ends. For example, the sheet defect may be undetected in a formed end which is subsequently used to seal a filled can. Such a defective end may result in an improper or inadequate seal of a can which does not manifest itself until pressure within the can from gas disassociated from the carbonated beverage has increased to the point to break the inferior seal and cause loss of carbonation or beverage. In some cases, such a leaker may not be discovered until the can has entered the distribution system. If the cause of the leak is then identified as resulting from a defect in the metal, any end made from the same coil or lot of metal may be suspect. The leaker may be only an anomaly, but even a small number traceable to a single coil or lot of material may result in the recall of all of the cans sealed with ends made from such coil or lot. It is apparent that the economic consequences of making defective ends can be out of all proportion to a small percentage of defective metal.
Many of the defects which arise during rolling of the sheet can be detected and corrections made to cure the defect. Because of the high rolling speeds, however, some defects may go undetected or it may be impractical to monitor the sheet as it is being produced to eliminate or minimize the defect. For example, methods are known to monitor the thickness of sheet as it is being rolled and, through a computer, change roll settings or other operating parameters to compensate for deviations from thickness. It is also known to monitor sheet as it is being rolled with television cameras to observe surface defects. Through a recorder or computer connection, the location and extent of the defect can be noted. Depending upon the nature and extent of the defect, the mill can be stopped and the defective material removed or the rolling can be continued and unwind the finished coil later to the extent necessary to remove the defective material. In any event, the control and/or removal of defects generated while rolling can be expensive and time-consuming to the sheet producer.
It is also known to monitor material being payed off of a coil to be processed in some fashion and stop the processing apparatus when a defect is noted. Abilock et al U.S. Pat. No. 3,786,265, for example, describes apparatus for detecting defects in a yarn sheet being fed to a warper. Two devices in tandem ahead of the warper monitor the yarn for defects. If both devices monitor the same defect, it is likely the defect is real and a signal may be sent for any desired purpose such as stopping the warper, for example. It is noted that the scope of the invention covers other coiled materials such as metal strips.
Heretofore, however, no process has been known for minimizing the amount of scrap in making a metal container by interrupting the operation of a cupper or end shell press to avoid making a defective cup or end shell while the passage of sheet continues through the press without interruption.